It is becoming apparent that new battery systems on which very strict requirements in respect of reliability are placed will increasingly be used in future, both in stationary applications (for example in wind power installations) and also in vehicles such as hybrid and electric vehicles. The background for these strict requirements is that failure of the battery can lead to failure of the entire system or even to a safety-critical problem. In wind power installations for example, batteries are used to protect the installation against impermissible operating states by virtue of rotor blade adjustment in strong winds.
In order to achieve the required power and energy data with the battery system, individual battery cells are connected in series and sometimes additionally in parallel.
During driving operation, the battery is typically connected to the vehicle by two contactors. The vehicle additionally contains a so-called intermediate circuit capacitor for cushioning voltage and current spikes. However, this intermediate circuit capacitor prohibits the two main contactors from being directly switched on since, in this case, a very large current would flow through the contactors into the intermediate circuit capacitor and could destroy it.
Therefore, a so-called precharging unit is generally used in the battery, said precharging unit first charging the intermediate capacitor with a limited current. The two main contactors are then connected only after the capacitor is charged.
FIG. 1 shows a basic circuit diagram of a precharging unit of this kind for charging an intermediate circuit capacitor from a battery according to the prior art. A large number of battery cells 11-1, . . . , 11-n are connected in series, and optionally additionally in parallel, in order to achieve a high output voltage and battery capacity which are desired for a respective application. A charging and disconnection device 14 is connected between a positive pole 27 of the battery cells and a positive battery terminal 12. A disconnection device 15 can optionally additionally be connected between a negative pole 28 of the battery cells and a negative battery terminal 13.
The positive battery terminal 12 and the negative battery terminal 13 are connections at which a load can be connected to the battery. By way of example, in a motor vehicle comprising the battery, the motor can be connected to the positive battery terminal 12 and the negative battery terminal 13 during driving operation.
The disconnection and charging device 14 and the disconnection device 15 each comprise a contactor 16 and, respectively, 17, which contactors are provided for the purpose of isolating the battery cells from the battery terminals 12, 13 in order to switch said battery terminals to zero potential. Otherwise, there is a considerable potential for danger to servicing personnel or the like on account of the high DC voltage of the battery cells which are connected in series. A charging contactor 18 with a charging resistor 19, which is connected in series to the charging contactor 18, is additionally provided in the charging and disconnection device 14. The charging resistor 19 limits a charging current for the intermediate circuit capacitor (not shown) when the battery is connected to the intermediate circuit of the vehicle (not shown). To this end, the contactor 16 is initially left open and only the charging contactor 18 is closed. If the voltage across the positive battery terminal 12 reaches the voltage of the battery cells, the contactor 16 can be closed and the charging contactor 18 can optionally be opened. The contactors 16, 17 and the charging contactor 18 considerably increase the costs of a battery since strict requirements are placed on the reliability of said contactors and on the currents which are to be carried by them.
The battery is charged either by means of a charging device being coupled to the vehicle intermediate circuit or by means of the charging device being connected to the battery by means of two additional contactors.
FIG. 2 shows a basic circuit diagram for coupling a charging device to a battery by means of two additional contactors according to the prior art. The charging device 20 is connected to the positive pole 27 of the battery cells by means of the contactor 21 and to the negative pole 28 of the battery cells by means of the contactor 22. These contactors 21, 22 also have to be designed such that they can be opened in the event of a possible short circuit on the charging device side. This is complicated and cost-intensive.